Handle Android stop capturer race.
Handle Android stop capturer race.

With the current order of stop capture processing on Android,
OnMemoryBufferFrame and OnTextureFrame may be called halfway through
Stop(). They must therefore check for the case of a null capturer_.

There used to be such checks, but they were accidantally removed in
commit #12895, cl https://codereview.webrtc.org/1973873003.

BUG=webrtc:5966
R=perkj@webrtc.org, sakal@webrtc.org

Review URL: https://codereview.webrtc.org/2033943004 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13031}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc